1. Field of the Invention
This invention relates generally to a method of recording holographic images and, in particular, to a method for recording a plural number of such holographic images into a recording material.
2. Description of the Prior Art
Optical holography, often called "wavefront reconstruction", may be described as the recording of a holographic image within a holographic recording material. Conventional holographic recording materials include silver halide emulsions, dichromated gelatin, thermoplastic media and time-dependent diffusion media.
A standard technique of producing a holographic image is by the interference of two coherent beams of light, respectively termed an "object beam" and a "reference beam". Both beams are generated from the same coherent source (typically a laser) to insure sufficient mutual coherence to form a stable interference pattern. To produce the holographic image the object beam is typically spatially modulated, e. g., it is reflected from a three-dimensional object. The reference beam is typically a non-spatially modulated coherent beam. The object beam and the reference beam are brought together (that is, spatially overlapped) at a predetermined position within a holographic recording material. It is the coherent addition of the wavefronts of the object beam and the reference beam within the material that constructs a holographic image within the recording material. In a similar manner holographic images may be created with multiple reference and object beams. The holographic image so constructed may also be referred to as the interference pattern, the holographic grating, the hologram, or simply, the image.
The term "multiplexed" holographic image refers to the construction of more than one holographic image within a holographic recording material. A variety of wavefront reconstruction options is permitted using multiplexed holographic images. Multiplexed holographic images in some cases make efficient use of the available area or volume of the recording medium. The holographic images produced by combinations of several reference and object beams may be constructed in the same physical region in the recording material. Alternatively, each of the holographic images may be constructed in a respective region of the recording material (spatial multiplexing).
Conventional methods used to multiplex a plural number N of holographic images into the same volume of the holographic recording material use two basic approaches: a simultaneous exposure technique or a sequential exposure technique. These two approaches are discussed in La Macchia and Vincelette, "Comparison of the Diffraction Efficiency of Multiple Exposure and Single Exposure Holograms", Applied Optics, Vol. 7, No. 9, pp. 1857-1858, September 1968) and in R. J. Collier et al., "Optical Holography", Academic press, Inc., 1971 and H. J. Caulfield, Ed., "Handbook of Optical Holography", Academic Press, Inc., 1979. These methods are particularly important for volume holography, where multiplexing N holograms at different angles (angle multiplexing) in the holographic recording material optimizes information recording density.
Simultaneous exposure requires that the optical beams constructing all N holographic images be generated simultaneously, so that only a single exposure of the holographic recording material is made. Perceived disadvantages of this approach include the difficulty in simultaneously generating beams for the N holographic images when N is large, and the problem of crosstalk between reference beams and object beams when constructing the N holograms.
Holographic exposure of conventional holographic recording materials, such as silver halide emulsions, dichromated gelatin, and thermoplastic media, involves recording a latent holographic image in the material. The final holographic image is obtained by a developing and fixing process. In the course of making multiplexed holographic images by sequential exposure, for example, the recording material may be left in the dark between exposures for extended periods of time while the recording apparatus is being modified. Subsequent exposures will simply add to the latent image that already exists.
Photopolymeric holographic recording materials are now known. Copending application Ser. No. 07/144,281, copending application Ser. No. 07/144,355, and copending application Ser. No. 07/144,840, all filed Jan. 15, 1988 and all assigned to the assignee of the present invention disclose and claim holographic recording materials. Holographic images are recorded in a photopolymer recording medium as a result of diffusion of unexposed monomer towards areas exposed with the highest intensity of incident light, creating a density gradient in the material which corresponds to the optical intensity gradient in the holographic image. After polymerization the higher density areas have a larger index of refraction than the lower density areas, thus forming a dielectric (phase) grating.
Identical serially recorded exposures, using typical exposure energies (ten milliJoules per square centimeter) recorded on the order of several seconds apart, are not recorded with equal strength in the material. This is due to the dynamics of the diffusion mechanism. Furthermore, monomer does not flow as readily to the second exposure as it does to the first, and not as much flows per unit of incident optical radiation.
When using a photopolymeric holographic recording material based on a time-dependent diffusion mechanism the material should not, in general, be left in the dark for extended periods of time (several seconds or minutes) between sequential exposures if one expects to achieve performance comparable to conventional holographic materials. Since in the holographic materials the latent image mechanism is replaced by a process involving the dynamic diffusion of molecules in the material the possibility of reciprocity failure exists. Reciprocity failure is due to the fact that the index of refraction change is not only a function of the irradiance-exposure time product, but also is dependent on the magnitude of the irradiance on the exposure time. In addition, the available modulation range (i.e., the possible index of refraction change) of the photopolymerizable holographic material varies with time because the diffusion mechanism and polymerization continues in the dark.
Conventional sequential multiple-exposure angle multiplexing has been demonstrated in current photopolymer formulations. However the resulting diffraction efficiencies are lower than predicted for this type of exposure, possibly due to the diffusion mechanism and its reversibility properties.
In view of the foregoing it is believed advantageous to provide a sequential exposure technique for multiplexing N holograms into a photopolymerizable holographic material which can overcome or minimize the dynamic effects of the time-dependent diffusion in the material.
Conventional appartus for forming sequential multiplexed holograms typically include a manual or automated (e.g., galvanometer controlled) mirror whereby the location and/or angle of incidence of at least either the object beam or the reference beam on the holographic recording material may be controlled. However, the degree of control over incident beam location and/or angle of incidence required in the formation of sequential multiplexed holograms is not a significant factor. So long as the mirror control system is able to place the incident beam within the region of the recording material dedicated to a given holographic image, the control system is sufficient for the purpose of forming sequential multiplexed holograms. Similarly, the beam positioning requirements placed on conventional apparatus for reading a sequential multiplexed hologram are not unduly stringent. So long as the reading beam is placed within the range of Bragg angle appropriate for the thickness of the holographic material, the angle between the object and reference beam and the wavelength of the light being used, the signal-to-noise ratio of the hologram is acceptable.
For reasons that become apparent hereinafter it is believed advantageous to provide an apparatus for forming holograms in which the position of either an object or a reference beam may be repeatably and precisely controlled.